1. Field of the Invention
The present invention relates generally to semiconductor fabrication, and more particularly, to apparatuses and methods for uniformly spin-coating chemicals over substrates used in the fabrication of semiconductor devices.
2. Description of the Related Art
The fabrication of a large variety of solid state devices such as semiconductors requires the use of planar substrates, otherwise known to those skilled in the art as wafers. An important consideration in the fabrication of these devices is the final number (i.e., yield) of functional dies remaining from each manufactured wafer. The functional dies are packaged, undergo testing (both electrical and otherwise), and substantially all of the packaged dies that pass the requisite testing are sold. It is therefore of utmost importance for manufacturers of these products to take advantage of economies of scale realized by increasing their production yield. Typically, depending on the individual die dimensions, upwards of 1000 or more dies may be fabricated on a single wafer. These wafers are typically on the order of six to twelve inches in diameter.
A typical fabrication process requires numerous steps, where several layers of material are cumulatively applied and patterned on the surface of the wafer. Once complete, these layers form the desired semiconductor structure necessary for the resulting circuit, or device. As can be appreciated, the final functional yield critically depends upon the proper application of each layer during the various process steps. Proper application of these layers typically depends, in turn, upon the ability to form uniform coatings of material on the surface of the wafer in an efficient, environmentally benign, and production worthy manner.
Various fabrication process steps implemented in making semiconductor devices utilize photolithography to define the desired patterns on the surface of a wafer. As is well known, photolithography is the process where light energy is applied through a reticle mask (using a stepper exposure camara) onto a photoresist material that is applied to the wafer to define patterns where subsequent etching will occur. These surface patterns represent a two dimensional layout of the desired structure that is fabricated on the surface of the wafer. It is therefore important that the photoresist material be applied in uniformly distributed coatings.
Conventionally, the application of photoresist coatings on the surface of a wafer is accomplished by casting a photoresist fluid on a wafer that is spinning at high speeds within a stationary exhausted bowl. In general, the stationary exhausted bowl is used to catch any excess fluids and remove particulates. Once the photoresist fluid is applied, the centrifugal force that result from the high rotational speed of the wafer overcomes the surface tension of the photoresist fluid, which causes the photoresist fluid to spread over the surface of the wafer.
A side effect of spinning the wafer is the inducement of air flows in the air immediately above and adjacent to the wafer surface. Unfortunately, this air flow tends to induce particles of photoresist to leave the wafer surface at the wafer's edge. When the photoresist leaves the wafer's edge, the free floating photoresist particulates have the potential to back contaminate the remainder of the wafer surface where a fresh coating of photoresist has just been applied. Although these particles may be removed by an exhaust system that may be part of the stationary bowl, the exhaust has the undesirable effect of drying out photoresist solvent films unevenly and, thus, producing a non-uniform coating of photoresist over the surface of the wafer during the spinning process. Back side contamination of the photoresist film with photoresist particulates and uneven drying of the photoresist film are therefore, undesirable yield reducing side effects of conventional spin-coating processes.
Another problem associated with conventional spin coating methods is photoresist fluid beading at the outer edge of the spinning wafer. Specifically, it is believed that surface tension and adhesion of the photoresist film to the wafer surface experienced during spinning causes the photoresist to from a "zone of increased thickness" at the edge of the wafer. This beading can typically contribute to a significant loss in functional devices that lie at and near the outer edge of the wafer.
Yet another problem associated with the beading effect at the edges of the wafers is that wafers are commonly stored in cassettes and, the increased thickness at the edges has the unfortunate effect of fracturing the wafers when they come in contact with the storage cassette. Of course, when wafers are fractured in any way, an instant loss in yield is experienced.
FIG. 1 is an illustration of a conventional open bowl apparatus 10 for spin coating a wafer 16. A wafer 16 is typically placed upon a rotatable chuck 14 which is rigidly connected to a spinning motor unit 20 by a shaft 18. As shown, the spinning chuck 14 and the wafer 16 are located within an open stationary bowl 12. The open stationary bowl 12 may include suitable exhaust passages 22 that are used to purge out particulates that may be produced within open stationary bowl 12 during a spin coating operation. During a spin coating operation, a number of air flows 17 may be produced in the air immediately above and adjacent to the spinning chuck 16. As described above, the air flows 17 tend to produce topographical variations in the applied coatings that unfortunately reduce yield.
In view of the foregoing, there is a need for apparatuses and methods that facilitate the application of substantially uniform spin coated materials over the surface of a wafer.